1. Field of the Invention
The present invention relates to a data transmission system for performing data transmission among a plurality of nodes connected thereto and, more particularly, to a data transmission system suitable for use as a LAN (local area network).
2. Description of the Prior Art
An LAN connecting a plurality of business machines such as word processors, office computers or printers through a low-cost and simple communication network recently has been receiving a great deal of attention and is now increasingly found in practical use.
Such a LAN must satisfy the requirements of easy installation, easy expansion/reduction of the network, and compatibility with as many different types of machine as possible. Therefore, if an existing computer network is used for this purpose, all these requirements cannot be satisfied at the same time and high data transmission efficiency cannot be achieved.
A token passing system is one type of communication control system used for such LANs. According to this system, a special packet called a "token," which represents a logical communication right to the network, is sent from one node to another connected to the network such that only the node which has acquired this token has access to the communication right. According to this token passing system, only a single node can send the desired data within a single time slot. For this reason, a communication disturbance, i.e., a "collision" or "contention", never occurs.
This token passing system is further divided into the token ring system and the token bus system in accordance with a difference in the exchange or assignment method of passing the token.
Acccording to the token ring system, the token is sequentially assigned to physically adjacent nodes. In contrast, according to the token bus system, the token is sequentially assigned to logically adjacent nodes, that is, to nodes having the closest node (logical) addresses.
For the reason described above, the shape of the network is limited to a ring network in the token ring system. However, the shape of the network can be a ring or bus (multidrop) network in the token bus system.
In the token bus system, when a token is to be sent, a destination address must be added to the token. Therefore, the sending origin must retrieve the address of a node which is to receive the token. This retrieval processing will be referred to hereinafter as "adjacent node retrieval processing".
In a network control system utilizing the conventional token bus system, once an adjacent node address is discriminated, the adjacent node retrieval processing is not performed in subsequent communication assignment processing, and the communication assignment processing is performed thereafter in accordance with the destination address which is already determined. However, when a node having the destination address becomes inoperative due to power failure or the like during operation of the network and another node is rendered operative in place of this inoperative node, the adjacent node retrieval processing is executed after completing every predetermined number of communication right assignment processing operations so as to retrieve the new adjacent node.
In the conventional case, the predetermined number of communication right assignment processing operations, after which the adjacent node retrieval processing is performed, is fixed so that the system is subject to the following disadvantages.
(1) When the predetermined number is relatively small, the frequency of executing the adjacent node retrieval processing operations during communication becomes high, so that the communication efficiency is low.
(2) When the predetermined number is relatively great, the frequency of the adjacent node retrieval processing performed during communication is low, so that it takes a long period of time before a token is received by a node which has been added to the network.
In a token bus system, the number of nodes connected in the network does not generally coincide with the number of addressable nodes. When the actual number of nodes is small, unncessary node retrieval processing is repeatedly performed to decrease the transmission efficiency.
In a data control system utilizing a conventional token bus system, once an adjacent node address is discriminated, node retrieval processing is not used for the subsequent communication assignment processing, and communication assignment processing is performed in accordance with the discriminated destination address. However, when the node having this destination address is rendered inoperative due to power failure or the like during operation of the network, normal communication right assignment cannot be performed. Then, the node retrieval processing is performed for the first time at this point for retrieval of the next adjacent node. This processing will be referred to hereinafter as a "downstream node fault recovery processing". However, if the addresses of nodes constituting a network are greatly spaced apart, the down stream node fault recovery processing time is long, thus decreasing the communication efficiency of the network.
When a node which has acquired a communication right wants to send data to another node, a corresponding destination address is generally added to the data to be sent. In order to determine the destination address, some means must be used to recognize the system configuration within the network. In other words, each node must be able to discriminate the addresses of the remaining nodes in the network.